The first is a direct question, where the questioner doesn't know what answer to expect.
The second is rhetorical -- i.e. it is the statement "I know you want some drugs" disguised as a question.
Addition
It just occurred to me that when spoken, the meaning of both can depend on emphasis. For example, these:
"Do you want some drugs?"
and
"Do you not want some drugs?"
are both genuine questions (i.e. not rhetorical) and could both mean that the person being asked had initially led the questioner to think that they wanted drugs, but had then given the opposite impression, leaving the questioner confused. And in fact, the second form in the cartoon shows the same effect with the emphasis as follows (although the cartoon's joke does not rely on this particular emphasis effect):
"Do you not want some drugs?"
The effect is easier to see if you speak any of the above alound, imagine yourself as the confused questioner, and add the words "So" to the start and "then" to the end of each.